This invention relates to a sensor for determining the concentration of a gaseous component in a fluid by absorption and, in the preferred embodiment, to a fiber optic probe for making in vivo measurements of the partial pressure of carbon dioxide in the blood.
Blood gas analysis is performed on nearly every hospital patient both during and after surgery. The analysis is concerned primarily with three parameters--the partial pressures of oxygen and carbon dioxide, the PO.sub.2 and PCO.sub.2, and the negative logarithm of hydrogen ion activity, the pH. These three parameters give a good indication of the patient's cardiac, respiratory and circulatory functioning and of metabolism. Monitoring the level of carbon dioxide in the blood alone gives a good indication of proper functioning in all of these systems because carbon dioxide, a waste product of metabolism, travels through the circulatory system and is removed through the respiratory system.
Several sophisticated blood gas analyzers are commercially available for analyzing blood samples after the blood is extracted from the patient (in vitro). The withdrawal and subsequent analysis of a blood sample is both cumbersome and time-consuming and does not allow for continuous monitoring of the concentrations of gases dissolved in a patient's blood. There has been a need for many years for a system which would enable blood gas measurements to be made directly in the patient (in vivo), thereby avoiding the difficulties and expense inherent in the in vitro techniques.
Among the suggestions in the prior art was the use of indwelling electrode probes for continuous monitoring of the blood gas. The in vivo electrode probes have not been acceptable. Two principal disadvantages of the electrode probes are the danger of using electrical currents in the body and the difficulty in properly calibrating the electrodes.
Also among the suggested techniques for in vivo measurement have been the use of fiber optic systems. In a fiber optic system, light from a suitable source travels along an optically conducting fiber to its distal end where it undergoes some change caused by interaction between the light and a component of the medium in which the probe is inserted or interaction with a material contained in the probe tip which is sensitive to (i.e., modified by) a component in the medium. The modified light returns along the same or another fiber to a light-measuring instrument which interprets the return light signal.
Fiber optic sensors appear to offer several potential benefits. A fiber optic sensor is safe, involving no electrical currents in the body. The optical fibers are very small and flexible, allowing placement in the very small blood vessels of the heart. The materials used, i.e., plastic, metal, and glass, are suitable for long-term implantation. With fiber optic sensing, existing optical measurement techniques could be adapted to provide a highly localized measurement. Light intensity measurements could be processed for direct readout by standard analogue and digital circuitry or a microprocessor. However, although the potential benefits of an indwelling fiber optic sensor have long been recognized, they have not yet been realized in a viable commercial product. Among the principal difficulties has been in the development of a sensor in a sufficiently small size which is capable of relatively simple and economical manufacture so that it may be disposable.
One type of in vivo fiber optic blood gas sensor proposed in the prior art involves the transmission of light directly into the blood. Light travels down the fiber and is allowed to leave the fiber at the distal end to interact directly with the blood and to report back via the return signal some characteristic spectroscopic property of the blood. An absorption sensor of this type is described in U.S. Pat. No. 4,509,522 to Manuccia et al., wherein absorption occurs as an incident light beam travels through the blood flowing between the ends of two chopped fibers, or as the beam travels through the blood flowing between the distal end of a fiber and a spaced mirror. These devices are complex and difficult to manufacture.
A similar fiber optic sensor for measuring the concentration of bilirubin in the blood is described in Coleman et al., "Fiber Optic Based Sensor For Bioanalytical Absorbance Measurements," 56 Anal. Chem. 2246-2249 (1984). The authors state that a sample chamber having a well-defined optical path length is necessary to obtain true absorbance values, and they propose a sensor having a single optical fiber in order to achieve a very small size. The Coleman et al. sensor consists of an optical fiber disposed in a needle and spaced from the distal end thereof, an aluminum foil reflector disposed at the end of the needle, and apertures in the needle to allow blood to flow into the chamber defined within the needle between the distal end of the fiber and the reflector. The needle and fiber assembly is then inserted into a larger gauge needle. Again, this device is difficult to construct and use of the rigid needle prohibits advancing the same through the blood vessels.
In another type of proposed blood gas sensor, the gas component to be measured is separated from the blood while making the spectroscopic determination. A gas-permeable membrane is used to form a chamber at the distal end of the fiber. The spectroscopic determination is made within the chamber either directly with the diffused component or with an intermediate reagent contained in the chamber. For example, U.S. Pat. No. 4,201,222 to Haase describes an optical catheter having at its distal end an absorption chamber in which a direct absorption measurement is made, formed by a cylindrical housing and a distensible semipermeable diaphram. The diaphram is silicon [sic]rubber which permits diffusion of oxygen and carbon dioxide into the chamber. A reflective coating of vacuum deposited or evaporated gold or aluminum is applied to the interior surface of the diaphram to prevent light from escaping. Two sources of light, one visible red for absorption by oxygen and the other infrared for absorption by carbon dioxide, are alternately pulsed down the fiber. The resiliently deformable diaphram also allows monitoring of blood pressure and pulse rate. Again however, the rigid sample chamber is difficult to construct in small size.
A fiber optic PCO.sub.2 sensor having an intermediate reagent is described in G. Vurek, et al., "A Fiber Optic PCO.sub.2 Sensor," 11 Annals of Biomedical Engineering 499-510 (1983). The sensor is made with plastic fibers and has at its distal end a silicone rubber tube filled with a phenol red-KHCO.sub.3 solution. The ambient PCO.sub.2 controls the pH of the bicarbonate buffer solution which influences the optical transmittance of the phenol red. The resulting electrical signal is said to be linearly related to the PCO.sub.2 over a certain range. However, a problem with shifts in the calibration curve is noted due to deformation of the flexible silicone tube.
A PO.sub.2 sensor probe utilizing a fluorescent intermediate reagent is described in U.S. Pat. No. 4,476,870 to Peterson et al. The probe, which operates under the principle of oxygen quenching of dye fluorescence, includes two plastic fibers ending in a section of porous polymer tubing serving as a jacket for the fibers. The tubing is packed with a fluorescent light-excitable dye placed on a adsorptive polymeric beads. The polymeric adsorbent is said to avoid the problem of humidity sensitivity found with inorganic adsorbents such as silica gel. Again, it is difficult to construct this jacket and bead configuration in a small size.
Still another approach is suggested in U.S. Pat. Nos. 4,399,099 and 4,321,057, to Buckles. In Buckles apparatus for biochemical analysis, the optical fiber itself serves as the medium in which the spectroscopic change occurs. The fiber, which is permeable to the blood gas of interest, absorbs the gas. The absorbed gas affects the light that emerges from the exit end of the fiber in proportion to the concentration of the gas in the sample fluid in contact with the fiber. However, because the fiber is permeable to the gas there is no way to control the path length over which absorbance occurs. Even if a nonpermeable coating covers all but a fixed portion of the fiber, there is no way to prevent the analyte from permeating along the length of the fiber thereby creating an indeterminate length of measurement. Without knowledge of the path length, an accurate absorbance measurement cannot be made. In other embodiments, the spectroscopic change occurs in one or more sheaths surrounding the optical fiber which may contain an intermediate reagent.
Thus, in spite of the great need for an in vivo fiber optic sensor, none of the proposed sensors has met with commercial success. Generally, they are either not reliable or not adapted for production manufacturing techniques. Devices involving sample chambers are difficult if not impossible to make in a miniature size required for use in the blood vessels. Other sensor probes are not flexible enough to be threaded through the narrow blood vessels.
It is an object of this invention to provide an in vivo sensor for the continuous real time monitoring of the concentration of a gaseous component in a body fluid, such as the carbon dioxide concentration of the blood.
It is another object of this invention to provide a very small and flexible fiber optic catheter that can be easily advanced through the small blood vessels and cavities of the body.
Still another object is to provide a fiber optic sensor which is both reliable and adapted for production manufacturing techniques.
A still further object is to provide a sensor having a fixed path length for absorption in order to obtain reliable absorbance measurements.